Various reconnaissance systems are used to intercept radar signals and to decipher their characteristics and angle-of-arrival (“AOA”). For example, a microwave intercept receiver may be used for this purpose. In Electronic Warfare (“EW”) related reconnaissance applications, the receiver is typically designed to fulfil roles including radar warning, electronic support measures (“ESM”), and Electronic Intelligence (“ELINT”). (For a related discussion, see J. B. Y. Tsui, Microwave Receivers With Electronic Warfare Applications, Wiley, New York, 1986, which is incorporated herein by reference.) In most conventional approaches, the intercept receiver is designed to perform two functions; the first function is to measure the characteristics of an intercepted signal and the second function is to determine the intercepted signal's AOA for the purpose of direction-finding (“DF”) and radar source locating.
With the proliferation of radar systems and the increasing number of radars employing complex waveform modulation, it is difficult to differentiate and sort out intercepted radar signals using coarse conventional parameters alone. Typically, these coarse conventional parameters include AOA, carrier frequency, pulse width (“PW”), pulse repetition interval (“PRI”), and scan pattern. Since many radars have similar conventional parameters, when signals having similar characteristics are compared using these coarse conventional parameters, ambiguity may occur in the sorting, classification, and identification processes.
One type of receiver which can be used to precisely measure conventional parameters, as well as intrapulse modulation, is the intrapulse receiver. However, the use of Low Probability of Intercept (“LPI”) radars in recent years with low-peak power has introduced a further requirement for modem intercept receivers. Modem receivers now require a much higher sensitivity in order to detect LPI radar signals. (In this respect, see Jim P. Y. Lee, Interception of LPI Radar Signals, Defence Research Establishment Ottawa, November 1991, NTIS AD A 246315, which is incorporated herein by reference.)
Until recently, most radars were designed to transmit short duration pulses with relatively high peak power. This type of signal is easy to detect using relatively simple, traditional EW intercept receivers making the attacker (i.e. radar source) vulnerable to either antiradiation missiles or Electronic Counter Measures (“ECM”). However, by using LPI techniques, it is possible to design a LPI radar that is effective against traditional intercept EW receivers. One of the most important LPI techniques is the use of phase or frequency waveform coding to provide transmitting duty cycles approaching one. This technique can result in drastic reductions in the peak transmitted power while maintaining the required average power.
Therefore with an increasing number of radars employing complex waveform modulation in addition to using low peak-power LPI signals, modem intercept receivers are required to perform the following three basic functions: (a) precisely measure and characterize conventional pulsed radar signals; (b) detect and characterize LPI signals; and, (c) determine AOA for both conventional pulsed and LPI radar signals. These three basic functions must be performed well in a multiple signal environment. In addition, modern receivers are also required to operate in the presence of interfering signals while providing signal detection at close to 100% Probability-of-Intercept (“POI”). In order to achieve these desirable operational requirements, the modern receiver must meet the following criteria: (i) a large instantaneous dynamic range (e.g. at least 60 dB); (ii) a large instantaneous frequency coverage (e.g. at least 1 GHz); (iii) a 360° instantaneous field-of-view; (iv) a good receiver sensitivity; and, (v) high immunity to interfering signals. Moreover, these requirements must be achieved with a minimal amount of hardware and at low cost.
Receiver architectures employing a mix of microwave, optical, and digital technologies are currently used to achieve the three basic receiver functions described above. There are disadvantages with these receiver systems in that the use of different receiver technologies results in a more complex and costly system architecture and related implementation. Since each receiver performs one specific function, elaborate control and correlation of the different receiver outputs is required to achieve a complete picture in a high signal density environment. If correlation is not performed precisely, errors may occur in the signal recognition process. Furthermore, the original signal content from each antenna is usually not preserved during the detection process as not all of the input data is in digital format.
To address this disadvantage, fully digital receivers have been proposed. If the original signals are preserved, then the same signal appearing on other channels may be optimally processed to enhance overall signal-to-noise ratio (“SNR”) during the extraction of signal characteristics. Digital receivers also have advantages relating to cost reduction, physical properties, and maintainability. With digital receivers, the trend has been to push the digital interface as close to the antennae as possible through the use of emerging analog-to-digital converter (“ADC”) capabilities. With the advent of high-speed ADCs and digital signal processing technologies, a multi-function receiver may be implemented using a multi-channel digital receiver architecture.
A multi-channel digital receiver architecture, where all three basic functions (as described above) can be performed simultaneously from one complete set of digital data, has recently been proposed by Lee (see U.S. Pat. No. 6,313,781, which is incorporated herein by reference). Referring to FIG. 1, there is shown a block diagram illustrating a multi-channel, multi-function digital intercept receiver architecture 10 in accordance with Lee's proposal. The receiver architecture 10 consists of M channels, each channel comprised of an antenna 12 for receiving an incoming radar signal 22 at an AOA θ 21 from a predetermined angle axis 26, one of M down converters 14, a local oscillator (“LO”) 16 signal 15, and an analog-to-digital converter 18. The receiver architecture 10 further includes a digital processor 20 for processing the digitized data from each of the M channels and for controlling the ADC 18 and down converters 14. Each antenna 12 in the array corresponds to a channel of the receiver architecture 10 and is comprised of a respective down-converter 14, which is driven by the local oscillator signal 15 to convert and amplify (i.e. gain K) the intercepted signal 22 from its respective antenna 12 to an intermediate frequency (“IF”) signal. Each intermediate frequency signal is fed to a respective ADC 18, which converts the intermediate frequency signal to a digital signal which is in turn applied to the digital processing system 20 for determining the relevant parameters from all the channels. In addition, a circuit which can enhance the detection of LPI signals and at the same time suppress strong conventional pulses, has also been proposed by Lee (see U.S. Pat. No. 6,388,604, which is incorporated herein by reference).
While the receiver architecture proposed by Lee and using digital processing technology is ideal for performing a multi-function role, current digital receivers are still limited in the instantaneous dynamic range and bandwidth that they can achieve due to the relatively poor performance of currently available high sampling rate ADCs. (For a selection of currently available ADCs, see the web pages of Thomson-CSF, Maxim Integrated Products, and Acqiris Data Conversion Instruments at http://www.tcs.thomson-csf.com, http://www.maxim-ic.com, and http://www/acqiris.com, respectively.) In fact, there is a considerable gap between military requirements and the current state-of-the-shelf technology. (On this point, see Larson (L. Larson, High Speed Analog-to-Digital Conversion with GaAs Technology: Prospects, Trends and Obstacle, 1988 IEEE International Solid-State Circuit Conference Technical Digest, pp. 2871–2878 (1988)) and Walden (R. Walden, Analog-to-Digital Converter Technology Comparison, Proceedings 1994 IEEE GaAs IC Symposium, Vol. 16, pp 217–219 (1994)), which are incorporated herein by reference.) Continued development of high performance components is required to meet the stringent ADC performance specifications (e.g., dynamic range, bandwidth, power, and reliability, etc.) demanded by military system designers. Moreover, there are several fundamental factors that may limit the achievable dynamic range at high sampling rates (see Larson). Consequently, even if higher sampling rates can be achieved, ADC devices are expected to be expensive and bulky. Furthermore, with present architectures, if overall system performance is to be improved, a large number of parallel and expensive receiver channels will typically be required which will result in high implementation costs.
A need therefore exists for an improved multi-channel, multi-function digital intercept receiver that may be implemented economically. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.